An internal combustion engine such as a diesel engine or a lean-burn gasoline engine includes an exhaust gas cleaning system which is provided along an exhaust passageway to dean exhaust gas. The exhaust gas cleaning system includes a NOx reducing catalyst such as a NOx occlusion reduction catalyst or selective reduction catalyst (SCR catalyst) for reducing NOx in exhaust gases and a particulate trapping filter for trapping particulate materials (PM) in exhaust gases.
In relation to cleaning of NOx, there is a problem that the amount of NOx is increased temporarily when a vehicle with an internal combustion engine is accelerated from a low-speed and low-load operating condition. Namely, when the vehicle with the internal combustion engine is accelerated from a state in which the vehicle continues to be driven with the internal combustion engine run in the low-speed and low-load condition and the temperature of the NOx reducing catalyst of the exhaust gas cleaning system remains equal to or lower than the activating temperature, the amount of fuel injected increases due to the acceleration and the combustion temperature within cylinders is increased, whereby the amount of NOx discharged from the cylinders is increased temporarily in association with the acceleration. However, since the NOx reducing catalyst is not activated, much of NOx generated passes through the NOx reducing catalyst without being cleaned.
This problem becomes particularly serious with trucks driven in urban areas, because in these trucks, the temperature of exhaust gases at idling is low and the trucks continue to be driven in the low-speed and low-load driving condition which makes it difficult for the NOx reducing catalyst to be activated.
In the exhaust gas cleaning system which is also referred to as a post treating system, however, when attempting to improve the NOx cleaning rate by devising the configuration of the NOx reducing catalyst or improving the performance thereof, there is caused a problem that the configuration of an injection device for injecting a reducing agent for reducing NOx or the configuration of the catalyst becomes complex. Further, since an expensive noble or precious metal is used for the NOx reducing catalyst, when attempting to improve the NOx cleaning rate by increasing the amount of such a precious metal to be used, there is caused a problem that the costs are increased.
On the other hand, in treating to clean exhaust gases released from an internal combustion engine, as described, for example, in JP-A-2009-002179 and JP-A-2001-241321, a hydrocarbon adsorbing catalyst is known in which hydrocarbons (HC) in exhaust gases are physically adsorbed by a zeolite-based adsorbing catalyst when the temperature of the exhaust gases is low, while the hydrocarbons adsorbed are released when the temperature of the exhaust gases is high.